With the increasing use of personal computers in households in recent years, electronic still cameras capable of photographing an object and recording the image as digital files into personal computers are becoming increasingly popular. Recently, electronic still cameras having a CCD detector array with 2 million pixels or more (i.e., the UXGA standard) has made its appearance. These CCD detector arrays not only have a smaller single pixel size than those of conventional CCD detector arrays; in addition, the array size is also larger in area. Furthermore, the photographic lens of an electronic still camera in which is mounted a CCD having a high number of pixels is required to have a high-resolution because the size of a single pixel is very small. In order to maintain a minimum amount of received light for each pixel, a higher brightness is required than would be needed if the pixel size were larger. Further, as the array size of a CCD array becomes larger, a wider angle imaging lens is required in order to keep the overall length of the imaging lens short.
As with conventional photographic lenses used with electronic still cameras, lens systems which have a wide angle and yet have a comparatively simple construction with only a small number of lens elements usually have a large FNO, and it is difficult to satisfy the need for a high degree of resolution.
On the one hand, with electronic still cameras, in order to improve the light receiving sensitivity of photographic pixels attached to micro-lenses, a retro-focus type lens may be used for the image-forming lens. In addition to providing a lens with a long focal length, the back-focus length of this type of lens can be made long.
However, if use is made of this type of retro-focus type lens as the image-forming lens, distortion is easily produced by the negative power of the previous lens group. In addition, when a subject to be photographed is positioned at a very close distance, curvature of the image surface occurs easily. Given a construction wherein the optical flux is transmitted by a more forward lens group, and converged by a more rearward lens group, astigmatism and/or lateral color easily become too large, and it becomes difficult to correct these aberrations.
The present invention relates to a wide-angle lens and, in particular, to a video camera or an electronic still camera which forms an object image onto a fixed photographic element. The object of the present invention is to provide a wide-angle lens having a large back focal length that favorably corrects aberrations so as to obtain an image of high quality.